In gas discharge lamps such as HID (High Intensity Discharge) and UHP (Ultra-High Pressure) lamps, a bright light is generated by a discharge arc spanning the gap between two electrodes disposed in the lamp. Advances in lamp manufacture, fill gas composition, and electrode design have led to the development of short-arc and ultra-short-arc discharge lamps, in which the tips of the electrodes are separated in the discharge chamber of the lamp by a very short distance, for example one millimeter or less, and the arc that spans this separation is therefore also short, but of intense brightness. Such lamps are useful for applications requiring a bright, near point source of white light, for example in image projection applications.
However, because of the high temperatures that are reached during operation at high voltages, the electrodes of such a lamp are subject to changes, i.e. an electrode tip may burn back, or ‘structures’ may grow at one or more locations on the electrode tip at the point where the arc attaches to the tip. Such physical alterations to the electrode can result in fluctuations in the brightness of the arc, since the arc may become longer or shorter, leading to fluctuations in the light output (flux) of the lamp. In an image projection system, such alterations in the light flux may even be noticeable to the user, an effect which is evidently undesirable.
Therefore, a stable arc length is of utmost importance in projection applications. Maintaining the light flux in modern projectors ultimately means maintaining a short arc-length for prolonged times. Therefore, in many cases, dedicated lamp driving schemes are employed in an effort to maintain the arc length. These schemes often include sophisticated combinations of different current waveshapes and operating frequencies, designed so that alterations to the electrode tips are avoided where possible, or that the growing and melting of structures on the electrodes occur in a controlled manner, so that the arc length can be stabilized. Depending on the choice of lamp driving scheme, modifications to the electrode surface can take effect within short to very short timescales.
In any one driving scheme, the lamp is driven with a certain lamp current waveshape at a certain frequency. The current waveshape can include pulses that recur at certain intervals, for instance ‘anti-flutter’ pulses, and this waveshape is usually not changed for the duration of a driving scheme. A switchover at some point in time between driving schemes occurs when one or more parameters of the lamp driving scheme are altered, for example by changing the current waveshape or operating frequency. For instance, the amplitude or width of a current pulse can be altered, or the operating frequency can be suddenly increased or decreased by a considerable factor. This change may be triggered, for example, by an observed parameter of the lamp such as the lamp voltage approaching a certain threshold, or be initiated after expiration of a predefined time interval.
The environment in an operating gas discharge lamp can be regarded as unstable or volatile, largely due to the nature of the lamp filling and the high operating temperatures and voltages. For instance, even during ‘steady’ operating conditions, the lamp voltage can be subject to brief but extreme fluctuations. For this reason, lamp parameters such as the lamp voltage are generally measured at regular intervals, for example every few milliseconds, and any decision to correct or adjust the lamp driving, e.g. a decision to correct or adjust the driving current, is usually based on a mean or average value of the observed input parameter, for example the lamp voltage, in a closed-cycle power control loop. An example is given by a PID (proportional-integral-derivative) controller, which attempts to maintain the lamp power at a certain target level such as the rated lamp power in response to an alteration in an input variable such as the value of the lamp voltage. Adjustment of the lamp power follows relatively slowly after the sudden change in lamp driving scheme. In this way, for example, glitches or peaks in the lamp voltage do not exert an immediate or direct influence on the lamp power, but are averaged into an overall value.
As mentioned above, advanced driving methods for short-arc lamps of the described types often include sudden changes of some parameter at well-known switching times. However, sudden changes such as an abrupt change of the lamp operation current waveshape or operating frequency resulting in an increase in the lamp voltage may lead to fluctuations in the light output of the lamp. The reason for this is that a closed-cycle power control of the lamp driver, based on a mean or average value of the observed parameter as described above, operates with a delay. As a result, the lamp power can exceed its rated or target value for a period of time, and the power overshoot can be observed as an increase in collected light flux. Changes in the light flux of such a lamp can be visible to a user of the application, and are therefore undesirable.
Therefore, it is an object of the invention to provide an uncomplicated way of stabilizing the light flux of a gas-discharge lamp of the type described above.